Substrate for reticle and method of manufacturing the substrate, and mask blank and method of manufacturing the mask blank

ABSTRACT

In a reticle substrate is used for forming a reticle held on a stepper and has main surfaces opposing each other, side faces, and chamfered surfaces formed between main surfaces and side faces, a flatness-measuring area is defined as an area excluding a peripheral area of a width of 3 mm inwardly laid from a boundary between the main surface and the chamfered surfaces and has a flatness of 0.5 μm or less, and a maximum height from a reference plane falls between −1 and 0 μm at the boundary between the main surface and the chamfered surface.

TECHNICAL FIELD

The present invention relates to substrates for reticles and relates tomethods for manufacturing the substrates. The reticles are mounted ontosteppers used in manufacturing semiconductor integrated circuits and areused as masks for reduced projection exposure apparatuses. The presentinvention also relates to mask blanks and relates to methods formanufacturing the mask blanks.

BACKGROUND ART

A reticle is mounted onto a stepper for the transfer of a pattern andused as a mask for a reduced projection exposure apparatus inmanufacturing of semiconductor integrated circuits. For example, thereticle has a pattern of a light-shielding film of chromium or the likeformed by sputtering, on a transparent glass substrate having at least amain surface mirror-finished.

Nowadays, as miniaturization of patterns progresses, the reticlesubstrate has been required to have a high flatness and high smoothness.

Recently, the reticle substrate is required to form a pattern with ahigh accuracy in position of the pattern in an area for the transfer ofthe pattern. For example, a substrate with 6025 size (6×6×0.25 inches)(1 inch=25.4 mm) is required to have a flatness of 0.5 μm or less (thesemiconductor design rule: 100 nm), more preferably, a flatness of 0.25μm or less (semiconductor design rule: 70 nm) at an area (referred to asa flatness-measuring area, hereinafter) of a main surface of thesubstrate excluding a peripheral area having a width of 3 mm from anedge or a boundary between the main surface and a chamfered surface. Theflatness is defined by a difference in height between the maximum andminimum values of a measuring face, relative to a virtual absolute plane(focal plane) calculated by least-squares method from the measuring faceof the substrate surface.

As described above, in the past, requirement of flatness has beendirected only to an area except a certain width from the side face ofthe substrate, i.e. only the central part of the substrate.

However, as miniaturization of patterns progresses, a line width of apattern has been recently reduced. Therefore, a peripheral configurationof the reticle substrate simply affects the pattern position accuracywhen a pattern on the reticle is transferred to a patterning substrateby using a stepper.

Namely, the reticle is generally attached to the stepper so that a mainsurface having the pattern faces a substrate on which the pattern istransferred. On this occasion, the reticle is fixed so as to secure abroad patterning area and to prevent misalignment of the substrate inthe operation of the stepper by vacuum chucking of the periphery outsidethe flatness-measuring area or the periphery spanning theflatness-measuring area and the area other than the flatness-measuringarea of the main surface of the substrate.

FIG. 2 shows a mechanism of attachment of the reticle to the stepper.

With reference to FIG. 2, a reticle 1′ is attached to substrate-holdingmembers 6, and is set to a substrate-holding unit 5. Thesubstrate-holding members 6 are disposed along two edges of the reticle1′ and are connected to a vacuum unit (not shown). The reticle 1′ isheld by suction of the vacuum unit.

On this occasion, if the end shape (flatness, edge flagging, etc.) atthe peripheries of the substrate (reticle substrate) constituting a basematerial of the reticle is improper, the substrate is deformed by vacuumchucking. Therefore, various problems take place in connection withpattern position accuracy in the transferred pattern, i.e. adisplacement in distances between the transferred patterns anddeterioration of uniformity of line width.

The above-mentioned problems are indicated in Proceedings of SPIEPhotomask and Next-Generation Lithography Mask Technology IX, Vol. 4754,43-53 (2002). Herein, if the edge of the reticle substrate curvesupward, the position accuracy decreases when the substrate is held bythe stepper The document suggests that the edge of the reticle substrateis preferably flat or has a little edge flagging.

The area of the main surface of the substrate supported by thesubstrate-holding members 6 is varied at every one of stepper makers.The amount of deformation of the substrate due to the vacuum chuckingvaries depending on this difference. Therefore, it is necessary that areticle substrate designed so that the amount of the substratedeformation due to the vacuum chucking is controlled within apredetermined value regardless of the substrate-holding members of thestepper apparatuses. However, it is difficult to actually responding tothis. Consequently, the amount of the substrate deformation due to thevacuum chucking must be controlled within a predetermined value,regardless of the areas supported by the substrate-holding members. Suchdesigned reticle substrate is necessary, but the document does notdisclose this point.

In general, the reticle substrate is manufactured by precise polishingdisclosed in Japanese Unexamined Patent Application Publication (JP-A)No. 1-40267.

In the precision polishing, both sides of a plurality of reticlesubstrates are polished at the same time, the so-called double-sidepolishing in batch is performed in multistage. The substrates arepolished with an abrasive containing cerium oxide as the mainingredient, and then are polished with an abrasive containing colloidalsilica as the main ingredient for the finishing. In the precisionpolishing, a suede-type polishing pad is used for smoothing the mainsurface of the substrates.

In the above-mentioned method, a high productivity is achieved, however,an excessive pressure is applied to the peripheries of the reticlesubstrate from the polishing pad. Therefore, the polishing pad sinks andinstability in the shape of the polishing pad occurs during thepolishing process. Thus, the reticle substrate having a high flatnesscannot be obtained. In particular, since the end shape of the reticlesubstrate becomes improper, the substrate cannot be reliably attached tothe substrate-holding means of the stepper. When the pattern on thereticle is transferred to a patterning substrate by using the stepper,the accuracy of pattern position is decreased. As mentioned above, someproblems exist.

Consequently, in Japanese Unexamined Patent Application Publication(JP-A) No. 2002-318450, proposal has been made about a method formanufacturing a glass substrate for a photomask. In the method, theglass substrate has a shape so as to have a flat face when the surfaceof the glass substrate is provided with a patterning of alight-shielding film. So, the exposure of the glass substrate can beperformed to the flat face. Such a glass substrate is prepared bypartial plasma etching depending on a difference calculated according toshapes of the glass substrate and a glass substrate as a raw material.

In the above-mentioned method, the exposed face of the glass substratetheoretically becomes flat during the exposure. However, since the faceroughness and a work-affected layer due to plasma etching occur on thesurface of the glass substrate, mechanical polishing must be performedwithin a very short time. Disadvantageously, a decrease in flatness dueto mechanical polishing for the very short time cannot be neglected, andthe additional processes reduce the productivity.

The peripheries of the substrate cannot be precisely measured by themethod (flatness measurement by optical interference method) formeasuring the shape of the substrate disclosed in Japanese UnexaminedPatent Application Publication (JP-A) No. 2002-318450. Therefore, evenif a desirable flat face is formed, the shape of the substrateperipheries cannot be actually formed. The reticle is deformed when thereticle is attached to the substrate-holding means of the stepper, andthe accuracy in position of the transferred pattern is problematicallydecreased.

Japanese Unexamined Patent Application Publication (JP-A) No. 2003-51472discloses a substrate having a flatness of 0.5 μm or less at theperipheral area having a width of 3 mm inward from the edges of the endfaces of the substrate. The purposes of this is to prevent a decrease intest sensitivity and to prevent a decrease in accuracy of the face towhich resist is applied. Namely, the above-mentioned Japanese UnexaminedPatent Application Publication (JP-A) No. 2003-51472 discloses onlyflatness at the peripheral area of the substrate and does not citeproblems when a thin film is formed on the substrate. JapaneseUnexamined Patent Application Publication (JP-A) No. 2003-51472 does notrefer to the flatness of areas other than the peripheral area of thesubstrate.

Therefore, if the reticle is configured by forming a thin film on thesubstrate defined by Japanese Unexamined Patent Application Publication(JP-A) No. 2003-51472, a large film stress is applied to the thin filmto deform the shape of the reticle. Furthermore, since the reliabilityof the flatness value measured at an area near the peripheral area ofthe substrate is low, deformation of the reticle cannot be sufficientlyprevented when the substrate-holding means of steppers are different bythe manufacturers.

DISCLOSURE OF INVENTION

The present invention has been achieved in view of the above-mentionedproblems, and provides a reticle substrate and a mask blank which canprevent deformation in a reticle prepared by forming a thin film formedon the substrate. Namely, it is an object of the present invention toprovide the reticle substrate and a method for manufacturing thesubstrate with a high productivity and at a high yield rate. The reticlesubstrate can control the deformation of the reticle and minimize adecrease in precision of a position of a transferred pattern, even ifsteppers have their respective substrate-holding members different inshape and the reticle is mounted on any one of the various shapes of thesubstrate-holding members which abut against the respective areas of thesubstrate.

Furthermore, it is an object of the present invention to provide themask blank and a method for manufacturing the mask blank with a highproductivity and at a high yield rate. The mask blank can control thedeformation of the reticle and minimize a decrease in precision of aposition of a transferred pattern, even if steppers have theirrespective substrate-holding members different in shape and the reticleis mounted on any one of the various shapes of the substrate-holdingmembers which abut against the respective areas of the substrate.

(Aspect 1) A reticle substrate comprising a pair of main surfacesopposing each other, two pairs of side faces that are right to the mainsurfaces and that are opposed in pair to each other, and chamferedsurfaces between the main surfaces and the side faces, wherein:

a flatness is not greater than 0.5 μm on a flatness measurement area ofeach main surface, from which an area of 3 mm laid inwardly from aboundary between the main surface and the chamfered surface is excluded,while the boundary between the main surface and the chamfered surfacehas a maximum height between −1 and 0 μm from a reference surface.

(Aspect 2) A mask blank comprising a thin film for a transfer pattern,on a main surface of the reticle substrate according to the aspect 1.

(Aspect 3) The mask blank according to the aspect 2, wherein the thinfilm has a film stress of 0.5 Gpa or less.

(Aspect 4) The mask blank according to aspect 2 or 3, wherein theflatness is not greater than 0.5 μm on a flatness measurement area ofthe main surface on which the thin film is formed, with an area of 3 mmwhich is laid inwardly from the boundary between the main surface andthe chamfered surface exempted from the flatness measurement area, whilethe boundary between the main surface and the chamfered surface has themaximum height between −1 and 0 μm from the reference surface.

(Aspect 5) A method for manufacturing the reticle substrate according toclaim 1, characterized by the steps of grinding and precisely polishinga main surface of a reticle substrate; thereafter measuring a surfaceconfiguration of the main surface on an area that includes a substrateperipheral portion supported by a substrate-holding member of anexposure apparatus; and modifying the surface configuration of the mainsurface on the basis of a result of the measurement so that the surfaceconfiguration of the main surface becomes a desired shape, by findingthat an area of the main surface is convex relative to an optionallydetermined reference plane, by providing, on the area, a pressure higherthan the other areas onto polishing pads of a polishing apparatus, witha polishing liquid being supplied towards the polishing pads, and bymoving the reticle substrate relative to the polishing pads.

(Aspect 6) The method for manufacturing the reticle substrate accordingto aspect 5, wherein the above-mentioned precisely polishing stepcomprises a roughly polishing process of removing surface defects of thesubstrate while maintaining the flatness obtained in the grindingprocess by using a relatively large abrasive grain; and amirror-polishing process of polishing the surface of the substrate formirror finish by using a relatively small abrasive grain.

(Aspect 7) A method for manufacturing the mask blank by forming a thinfilm as a transfer pattern on a main surface of the reticle substratemanufactured by the method according to aspect 5 or 6.

(Aspect 8) The method for manufacturing the mask blank according toaspect 7, comprising a heating process of suppressing that change of themaximum height from the reference plane which appears during or afterthe forming of the thin film, at the boundary between the main surfaceand the chamfered surface, the heating process being carried out beforeand after the forming of the thin film.

According to the above-mentioned aspect 1, the substrate has a flatnessof 0.5 μm or less at a flatness-measuring area defined as an area of themain surface of the substrate excluding a peripheral area of a width of3 mm inward from the boundary between the main surface and the chamferedsurfaces and the maximum height of −1 to 0 μm from a reference plane atthe boundary between the main surface and the chamfered surface.Therefore, deformation of the reticle can be controlled and a decreasein precision of a position of a transferred pattern can be minimized,even if steppers have their respective substrate-holding membersdifferent in shape and the reticle is mounted on any one of the variousshapes of the substrate-holding members which abut against therespective areas of the substrate.

The flatness in the present invention is a value indicating warpage ofthe substrate surface and is shown by Total Indicated Reading (TIR). Theflatness is a difference in height from a reference plane which isarbitrarily determined at the side of the front main surface of thesubstrate between the maximum point and the minimum point of a surfaceshape in the main surface (a difference in height between the maximumpoint and minimum point of a measuring face against a virtual absoluteplane (focal plane) calculated by least-squares method from themeasuring face).

The flatness can be measured by utilizing optical interference. Theoptical interference method utilizes that a difference in height of asurface of the substrate can be observed as a phase shift of a reflectedlight when the surface is irradiated with a coherent light such as alaser. In such a method, since the flatness cannot be precisely measuredat an area of a width of 3 mm inward from the boundary between the mainsurface and the chamfered surfaces of the substrate, the resultingflatness of this area is low in reliability. Therefore, this area isexcluded from the flatness-measuring area.

The flatness at the flatness-measuring area is preferably 0.5 μm orless, more preferably 0.25 μm or less, and most preferably 0.05 μm orless.

The substrates having the same flatness at the flatness-measuring areacan have different end shapes, i.e. flat toward the chamfered surfaces,curving downward (flagging-end or roll-off type) toward the chamferedsurfaces, or curving upward (ski-jump type) toward the chamferedsurfaces.

When the reticle is held to the substrate-holding member of a stepper ofan exposure apparatus by vacuum chucking, the vacuum chucking of theperiphery of the substrate causes deformation of the substrate.Therefore, the flat end or the flagging end (roll-off type) is morepreferable than the end curving upward (ski-jump type).

In the present invention, the above-mentioned end shape is evaluated bythe maximum height from a reference plane at the boundary between themain surface and the chamfered surface to quantify a degree of curvingdownward or upward.

The reference plane is appropriately adjusted based on the substratesize. For example, as shown in FIG. 3, the boundary or edge 14 betweenthe main surface 2 and the chamfered surface 4 is defined as a referenceposition. A plane or a line determined by connecting a point of 3 mminward from this reference position to a point of 16 mm toward thecenter from this reference point is defined as a virtual reference planeor a virtual reference line. When the virtual reference plane (orvirtual reference line) is assumed to have a height of 0, the degree ofcurving upward or downward is defined by the maximum height at theboundary between the main surface and the chamfered surface. The maximumheight is 0 when the end shape is flat. The maximum height is minus (−)when the peripheral area of the main surface of the substrate curvesdownward (flagging end or roll-off type). The maximum height is plus (+)when the peripheral area of the main surface of the substrate curvesupward (ski-jump type).

According to the present invention, the maximum height is between −1 μmand 0 μm. When the maximum height exceeds 0 μm, the periphery curvesupward. Consequently, deformation of the reticle increases when thereticle is mounted on the substrate-holding means of the stepper. Thiscauses a decrease in precision of the position of a transferred pattern.On the other hand, when the maximum height is less than −1 μm, thesubstrate-holding member of the stepper cannot tightly hold the reticle.Consequently, the holding is unstable. This causes a decrease inprecision of the position of a transferred pattern, which isundesirable. As miniaturization of a pattern progresses, the precisionof the position of the pattern has been required to be high. Therefore,the maximum height is preferably between −0.5 μm and 0 μm, morepreferably between −0.25 μm and 0 μm, more preferably between −0.1 μmand 0 μm, and most preferably between −0.05 μm and 0 μm.

A combination of the flatness at the flatness-measuring area and themaximum height from a reference plane at the boundary between the mainsurface and the chamfered surface is preferably a flatness of 0.25 μm orless and the maximum height of −0.5 to 0 μm, more preferably −0.25 to 0μm, from a reference plane at the boundary between the main surface andthe chamfered surface, and is more preferably a flatness of 0.05 μm orless and the maximum height of −0.1 to 0 μm, more preferably −0.05 to 0μm, from a reference plane at the boundary between the main surface andthe chamfered surface.

According to the above-mentioned aspect 2, a mask blank is prepared byforming a thin film functioning as a transfer pattern on the mainsurface of the reticle substrate described in aspect 1. This can controldeformation of the reticle and minimize a decrease in precision of aposition of a transferred pattern, even if steppers have theirrespective substrate-holding members different in shape and the reticleprepared by using the mask blank is mounted on any one of the variousshapes of the substrate-holding members which abut against therespective areas of the substrate.

According to the above-mentioned aspect 3, when the thin film has a filmstress of 0.5 Gpa or less, the flatness of the substrate and the maximumheight from a reference plane at the boundary between the main surfaceand chamfered surface in aspect 1 rarely change. Thus, the mask blankcan maintain the surface shape of the substrate. Therefore, the flatnessof the substrate and the maximum height from a reference plane at theboundary between the main surface and chamfered surface of the reticleprepared by using this mask blank rarely change. This can controldeformation of the reticle and minimize a decrease in precision of aposition of a transferred pattern when the reticle is mounted.Preferably, the film stress of the thin film is 0.2 Gpa or less, morepreferably, 0.1 Gpa or less. When the main surface of the reticlesubstrate is provided with a plurality of thin films, the film stressmeans the total film stress of the plurality thin films. The film stresscan be calculated from a difference in those of the substrate before andafter the thin film is formed on the reticle substrate.

According to the above-mentioned aspect 4, specifically, aflatness-measuring area defined as an area of the main surface having athin film of the mask blank excluding a peripheral area of a width of 3mm inward from the boundary between the main surface and the chamferedsurfaces has a flatness of 0.5 μm or less, and the maximum height from areference plane at the boundary between the main surface and thechamfered surface is between −1 μm and 0 μm. This can certainly minimizea decrease in precision of a position of a transferred pattern.

According to the above-mentioned aspect 5, the flatness of the surfaceshape of the substrate is modified (or adjusted) by mechanical ormechanochemical polishing using a polishing pad unlike a conventionalmethod performed by plasma etching. Consequently, as the surfaceroughness of the substrate is maintained or improved, the surface shapeis modified (or adjusted) to a desired surface shape. This can controldeformation of the reticle and minimize a decrease in precision of aposition of a transferred pattern when the reticle is mounted on thesubstrate-holding means of the stepper. Thus, the reticle substrateaccording to aspect 1 can be manufactured with a high productivity andat a high yield rate.

According to aspect 5, the modifying of the shape can be performed byusing a polishing apparatus which applies a plurality of pressing bodiesas described below to almost the entire main surface of the substrate.

The surface shape (flatness) of the main surface of the substrate ispreferably measured by a flatness-measuring apparatus utilizing opticalinterference in view of precision of the apparatus. The opticalinterference method utilizes that a difference in height of a surface ofthe substrate can be observed as a phase shift of a reflected light whenthe main surface of the substrate is irradiated with a coherent lightsuch as a laser.

The above-mentioned grinding process is usually performed to achieve (1)the adjusting of the thickness of a sliced substrate to a predeterminedone, (2) the uniformizing of a work distortion layer, and (3) theregulation of the flatness.

In the grinding process, generally, both faces of the reticle substrateheld by a carrier are ground by rotating top and bottom tables and thecarrier under the supplying of an abrasive grain liquid to both faces ofthe reticle substrate. Such double-side grinding is usually performed bytwo stages of rough grinding and precision grinding. For example, in therough grinding, abrasive grain such as silicon carbide or alumina ofabout #400 to #600 is used, and in the precision grinding, abrasivegrain such as alumina or zirconia of about #800 to 1500 is used.

The above-mentioned precision polishing process is usually performed toachieve mirror finish while the flatness obtained in the grindingprocess is maintained or improved.

In the precision polishing process, generally, both faces of the reticlesubstrate held by a carrier are polished by rotating the carrier and topand bottom tables on which polishing pads are adhered under thesupplying of slurry containing abrasive grain to both faces of thereticle substrate. The polishing pads and slurry used in the double-sidepolishing are appropriately adjusted according to the substratematerial, the surface shape to be prepared, and the surface roughness.

Examples of the polishing pads include a hard polishing pad such as aurethane pad, and a flexible polishing pad such as a suede-type pad.

The polishing liquid is comprised of at least one polishing abrasiveselected from cerium oxide, ziconium oxide, aluminum oxide, colloidalsilica, and the like; and a solvent selected from water, alkalis, andacids.

The abrasive having an average particle size of several tens nm to about1 μm is used according to the surface roughness to be obtained.

According to the above-mentioned aspect 6, the precision polishingprocess is performed in multistage of a rough polishing process and amirror-polishing process. The rough polishing process is for removingsurface flaws of the substrate by using abrasive grain having arelatively large particle size, while maintaining the flatness obtainedin the grinding process. The mirror-polishing process is for polishingthe surface of the substrate for mirror finish by using abrasive grainhaving a relatively small particle size. Thus, the reticle substrateaccording to aspect 1 can be manufactured with a high productivity.

When the reticle substrate is made of glass, the polishing abrasivegrain used in the rough polishing process is preferably cerium oxidehaving an average particle size of about 1 to 2 μm, and the polishingabrasive grain used in the mirror-polishing process is preferablycolloidal silica having an average particle size of several tens to 100nm.

According to the above-mentioned aspect 7, the mask blank according toaspect 2 can be manufactured by forming a thin film functioning as atransfer pattern on the main surface of the reticle substrate obtainedby the method of manufacturing the reticle substrate according to aspect5 or 6.

According to the above-mentioned aspect 8, the film stress of the thinfilm is decreased by performing a heating process during or after theforming of the thin film. Consequently, the flatness of the substrateand the maximum height from a reference plane at the boundary betweenthe main surface and the chamfered surface rarely change. Thus, the maskblank maintaining the surface shape of the substrate can bemanufactured.

Materials and sizes of the reticle substrate according to the presentinvention do not have any limitation.

As the material of the reticle substrate, any material can be used aslong as the material is transparent against the exposure light of theexposure apparatus. Typical material is glass. Examples of the glassinclude synthetic quartz glass, soda lime glass, aluminosilicate glass,aluminoborosilicate glass, non-alkali glass, and crystallized glass.

In general, the size of the reticle substrate is 6025 size(152.4×152.4×6.35 mm) or 5009 size (127×127×2.29 mm), but is not limitedto these sizes.

The thin film functioning as a transfer pattern according to the presentinvention causes an optical change of the exposure light used when thepattern is transferred. Examples of the thin film include thelight-shielding film and a phase-shift film.

In the mask blank, a resist film may be formed on the thin filmfunctioning as the transfer pattern.

The reticle substrate and the mask blank according to the presentinvention can control deformation of the reticle and minimize a decreasein precision of a position of a transfer pattern when the reticle ismounted on the substrate-holding means of the stepper.

According to the method for manufacturing the reticle substrate and maskblank of the present invention, a reticle substrate can be manufacturedwith a high productivity and at a high yield rate. The reticle substratecan control deformation of the reticle and minimize a decrease inprecision of a position of a transfer pattern when the reticle ismounted on the substrate-holding means of the stepper.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall view of a reticle substrate according to thepresent invention.

FIG. 2 is a diagram showing a mechanism of a stepper for fixing thesubstrate.

FIG. 3 is a diagram showing a shape of a periphery of the reticlesubstrate according to the present invention.

FIG. 4 is a plan view of a substrate-polishing apparatus used in thepresent invention.

FIG. 5 is a cross-sectional view of a substrate-polishing apparatus usedin the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

With reference to FIG. 1, a reticle substrate 1 used in this Example wasa square-shaped substrate having a pair of main surfaces 2 opposing eachother, two pairs of side faces 3 right to the main surfaces 2, andchamfered surfaces 4 intervening between the main surfaces 2 and theside faces 3. The substrate was made of a synthetic quartz glass having6025 size of 152.4×152.4×6.35 mm.

A flatness-measuring area was defined as an area of a main surfaceexcluding a peripheral area of a width of 3 mm inward from each boundarybetween the main surface 2 and the chamfered surfaces 4. Theflatness-measuring area had a flatness of 0.22 μm. When the area of themain surface surrounded by a line at 3 mm and a line at 16 mm inwardfrom the boundary between the main surface and the chamfered surfaceswas defined as a virtual reference plane (0), the end shape had themaximum height of −0.5 μm at the boundaries between the main surface andthe chamfered surfaces.

The above-mentioned flatness value was measured by an opticalinterference-type flatness-measuring apparatus (FM-200: Tropel Corp.),and the end shape was measured by a stylus-type roughness-measuringapparatus (SURFTEST-501: Mitutoyo Corp.).

Then, a substrate deformation-testing apparatus for vacuum chucking oftwo sides of the substrate as in the substrate-holding members of thestepper shown in FIG. 2 was prepared for conducting the deformation testof the substrate. The reticle substrate according to the presentinvention was held by vacuum chucking, and a change in flatness wasmeasured by an optical interference system (Zygo Mark GPI) to be 0.1 μmor less. Thus, deformation of the substrate was not almost observed.

A light-shielding film was formed on this reticle substrate bysputtering. The light-shielding film was a laminated film composed ofCrN/CrC/CrON, and the laminated film contained helium (He). Then, thesubstrate was heated at 120° C. for a predetermined time to prepare amask blank. The flatness of the resulting mask blank was measured at theabove-defined flatness-measuring area. The film stress of thelight-shielding film was calculated from the difference in the flatnessof the substrate before and after the forming of the light-shieldingfilm to confirm a stress of 0.1 Gpa or less, i.e. almost 0 Gpa. Themaximum height at the boundary between the main surface and thechamfered surface was −0.5 μm. Thus, the end shape of the substrate atthe side having the light-shielding film was the same as that before thelight-shielding film was formed.

The reticle, the substrate having the light-shielding film pattern, isprepared by using this mask blank. The pattern position had asatisfactory precision expected when an F₂ excimer laser exposure lightsource (exposure wavelength: 157 nm) was used, even if steppers hadtheir respective substrate-holding members different in shape and thereticle was mounted on any one of the various shapes of thesubstrate-holding members which abut against the respective areas of thesubstrate.

A method for manufacturing the reticle substrate will now be described.

The method for manufacturing the reticle substrate according to thepresent invention includes a grinding process that prepares a reticlesubstrate having edges chamfered surface at a predetermined size andthat grinds both main surfaces of the substrate; a rough polishingprocess that removes surface flaws or defects of the substrate whilemaintaining the flatness obtained in the grinding process; amirror-polishing process that polishes the surfaces of the substrate formirror finish; a surface shape-measuring process that measures thesurface shape of one main surface of the mirror-polished substrate; anda shape-modifying process that modifies the surface shape of thesubstrate by partially modifying the shape of the surface according tothe measured data so that the substrate has a desired surface shape. Themethod includes washing processes between the grinding process and therough polishing process, between the rough polishing process and themirror-polishing process, between the mirror-polishing process and theshape-modifying process, and after the shape-modifying process.

A polishing apparatus used in the shape-modifying process will now bedescribed.

FIG. 4 is a plan view of a mechanism of a polishing apparatus used formodifying the surface shape of the reticle substrate according to thepresent invention. FIG. 5 is a cross-sectional view taken along the lineA-A′ of FIG. 4.

With FIGS. 4 and 5, the reticle substrate is prepared by the grindingprocess and a precision polishing process. The reticle substrate is heldon a polishing table with a retainer ring 11, and is rotated under beingpressed against the polishing table 13 by pressing bodies 9. Theretainer ring 11 serves to make the pressure applied to the periphery ofthe substrate uniform by holding down the polishing pad 12. Thepolishing pad 12 is adhered on the polishing table 13. The main surfaceof the substrate is polished by rotating the pressing bodies 9 in theopposite direction of the rotation of the polishing table under pressingthe substrate against the polishing pad. A large number of the pressingbodies 9 are provided so as to cover the top surface of the substrate,and are held by a pressing bodies-holding means 10. Therefore, thepressure applied to the substrate can be partially controlled. Thepressing bodies 9 are driven by an air cylinder.

For example, when the reticle substrate after the precision polishingprocess has a concave shape of a flatness of 0.25 μm at aflatness-measuring area defined as an area of the main surface excludinga peripheral area of a width of 3 mm inward from the boundary betweenthe main surface of the substrate and the chamfered surfaces and the endshape curves upward and has the maximum height of 1 μm, loads areapplied only to the pressing bodies of the polishing apparatus placed atthe peripheries of the substrate so that the pressure is applied to onlythe peripheries of the reticle substrate. Thus, a flagging end shape of−0.5 to 0 μm can be readily obtained by modifying only the end shape.

When the reticle substrate after the precision polishing process had aconvex shape of a flatness of 1 μm at the flatness-measuring area and aflagging end shape of the maximum height of 1 μm, the surface shape ofthe flatness-measuring area and the periphery of the substrate must bemodified. The reticle substrate having a flatness of 0.5 μm or less andan end shape of the maximum height of −0.5 to 0 μm can be readilyobtained by applying larger loads to the pressing bodies positioned atthe central part of the substrate than that to the pressing bodiespositioned at the peripheries of the substrate.

Grinding Process

The grinding process was performed by grinding both faces by usingalumina abrasive grain of #400 and alumina abrasive grain of #800 asabrasive grain.

Rough Polishing Process

The rough polishing process was performed by polishing both faces byusing a polishing pad of foamed polyurethane and cerium oxide having anaverage particle size of about 1 to 2 μm.

Mirror-polishing Process

The mirror-polishing process was performed by polishing both faces byusing a flexible suede-type polishing pad and colloidal silica having anaverage particle size of about 100 nm.

The substrate was washed with a low concentration hydrofluoric acidsolution after each process.

Surface Shape-measuring Process

A flatness-measuring area defined as an area of the main surfaceexcluding a peripheral area of a width of 3 mm inward from the boundarybetween the main surface and the chamfered surfaces of the substrate hada concave shape of a flatness of 0.5 μm. The end shape had the maximumheight of 1 μm at the boundary between the main surface and thechamfered surface. The root-mean-square surface roughness (RMSroughness) of the main surface of the substrate measured with an atomicforce microscope was favorably 0.15 nm.

Shape-modifying Process

The periphery of the substrate was preferentially polished to form anend shape having a decline, and the flatness of the substrate at thecentral part excepting the periphery of the substrate was furtherimproved by applying a load of 0.5 kg/cm² to the pressuring bodiespositioned at the periphery of the substrate and applying a load of 0.1to 0.2 kg/cm² to the pressuring bodies positioned at the central part ofthe substrate. Thus, the shape was modified. Slurry containing colloidalsilica abrasive grain having an average particle size of about 100 nmwas used as a polishing liquid. The processing was continued till theend shape has the maximum height of −0.5 to 0 μm. Then, the substratewas washed with a low concentration hydrofluoric acid solution to obtainthe reticle substrate.

As a result, a flatness-measuring area defined as an area of the mainsurface excluding a peripheral area of a width of 3 mm inward from theboundary between the main surface and the chamfered surfaces of thesubstrate had a flatness of 0.22 μm. The end shape had the maximumheight of −0.5 μm at the boundary between the main surface and thechamfered surface.

A hundred reticle substrates were manufactured according to theabove-mentioned EXAMPLE. Each of the hundred reticle substrates had aflatness of 0.5 μm or less at the flatness-measuring area defined as anarea of the main surface excluding a peripheral area of a width of 3 mminward from the boundary between the main surface and the chamferedsurfaces of the substrate. The maximum height from the reference planewas −0.5 to 0 μm at the boundary between the main surface and thechamfered surface. The change amount of the flatness did not exceed 0.1μm, and no deformation of the substrate was observed.

Light-shielding films were formed on the reticle substrates bysputtering. Each light-shielding film was a laminated film composed ofCrN/CrC/CrON, and the laminated film contained helium (He). Then, thesubstrates were heated at 120° C. for a predetermined time to preparemask blanks. The flatness of each resulting mask blank was measured atthe above-defined flatness-measuring area. The film stress of eachlight-shielding film was calculated from the difference in the flatnessof the substrate before and after the forming of the light-shieldingfilm, and a stress of 0.1 Gpa or less, i.e. almost 0 Gpa was confirmed.The end shape of each substrate at the side having the light-shieldingfilm had the maximum height of −0.5 μm at the boundary between the mainsurface and the chamfered surface. Thus, the end shapes were the same asthose before the light-shielding films were formed.

Reticles, the substrates on which light-shielding film patterns wereformed, were prepared by using these mask blanks. Each pattern positionhad a satisfactory precision expected when an F₂ excimer laser exposurelight source (exposure wavelength: 157 nm) was used, even if steppershad their respective substrate-holding members different in shape andthe reticle was mounted on any one of the various shapes of thesubstrate-holding members which abut against the respective areas of thesubstrate.

EXAMPLE 2

A reticle substrate having a flatness of 0.5 μm at theflatness-measuring area defined as an area of the main surface excludinga peripheral area of a width of 3 mm inward from the boundary betweenthe main surface and the chamfered surfaces of the substrate and havingthe maximum height of −0.82 μm from a reference plane at the boundarybetween the flatness-measuring area and the peripheral area was preparedby a method similar to that in the EXAMPLE 1 under appropriatelyadjusted processing conditions.

The change in the flatness defined as in EXAMPLE 1 was measured toconfirm a flatness change of 0.1 μm or less. Thus, no deformation of thesubstrate was observed.

A mask blank was prepared by forming a light-shielding film on thereticle substrate by sputtering. The light-shielding film was alaminated film composed of CrN/CrC/CrON, and the laminated filmcontained helium (He). The flatness at the above-definedflatness-measuring area was measured. The film stress of thelight-shielding film was calculated from the difference in the flatnessof the substrate before and after the forming of the light-shieldingfilm to confirm a stress of 0.45 Gpa. The end shape of the substrate atthe side where the light-shielding film was formed had the maximumheight of −0.86 μm at the boundary between the main surface and thechamfered surface. The maximum height changed by 0.04 μm.

A reticle, the substrate on which a light-shielding film pattern wasformed, was prepared by using this mask blank. The pattern position hada satisfactory precision expected when an ArF excimer laser exposurelight source (exposure wavelength: 193 nm) was used, even if steppershad their respective substrate-holding members different in shape andthe reticle was mounted on any one of the various shapes of thesubstrate-holding members which abut against the respective areas of thesubstrate.

EXAMPLE 3

An MoSiN halftone film was formed on the reticle substrate of EXAMPLE 1by sputtering, and the substrate was heated at 120° C. for apredetermined time. Then, a light-shielding film made of a CrN/CrC/CrONlaminated film was formed on the MoSiN halftone film by sputtering toprepare a halftone-type phase-shift mask blank. The flatness of theresulting mask blank was measured at the above-definedflatness-measuring area. The film stress of the halftonefilm/light-shielding film was calculated from the difference in theflatness of the substrate before and after the forming of the film toconfirm a stress of 0.2 Gpa. The end shape of the substrate at the sidewhere the halftone film and the light-shielding film were formed had themaximum height of −0.82 μm at the boundary between the main surface andthe chamfered surface. Thus, the maximum height was the same as that ofthe reticle substrate before the films were formed.

A reticle, the substrate on which a halftone film pattern was formed,was prepared by using this halftone-type phase-shift mask blank. Thepattern position had a satisfactory precision expected when an ArFexcimer laser exposure light source (exposure wavelength: 193 nm) wasused, even if steppers had their respective substrate-holding membersdifferent in shape and the reticle was mounted on any one of the variousshapes of the substrate-holding members which abut against therespective areas of the substrate.

COMPARATIVE EXAMPLE

A reticle substrate was prepared by the same method as in theabove-described EXAMPLES except the shape-modifying process. Theshape-modifying method of the shape-modifying process was performed bypartial plasma etching based on the measured data in the surfaceshape-measuring process and mechanical polishing for a very short timein order to decrease the surface roughness of the glass substrate causedby the plasma etching.

As a result, the flatness at the flatness-measuring area defined as anarea of the main surface excluding a peripheral area of a width of 3 mminward from the boundary between the main surface and the chamferedsurfaces of the substrate was favorably 0.25 μm, but the end face hadthe maximum height of −1.5 μm at the boundary between the main surfaceand the chamfered surface.

Then, a deformation test of the substrate was performed as in above. Thechange in the flatness by vacuum chucking was 0.5 μm. Thus, thedeformation of the substrate occurred.

A mask blank was prepared by forming a light-shielding film of alaminated film composed of CrN/CrC/CrON on the reticle substrate bysputtering. Then, a reticle, the substrate on which a light-shieldingfilm pattern was formed, was prepared by using this mask blank. Thepattern position did not satisfy the precision expected when an F₂excimer laser (exposure wavelength: 157 nm) or an ArF excimer laser(exposure wavelength: 193 nm) exposure light source was used, when thesteppers had their respective substrate-holding members different inshape and the reticle was mounted on any one of the various shapes ofthe substrate-holding members which will abut against the respectiveareas of the substrate.

A hundred reticle substrates were manufactured according to theabove-mentioned COMPARATIVE EXAMPLE. Only 74 of the 100 reticlesubstrates had a flatness of 0.5 μm or less at the flatness-measuringarea defined as an area of the main surface excluding a peripheral areaof a width of 3 mm inward from the boundary between the main surface andthe chamfered surfaces of the substrate and the maximum height of −0.5to 0 μm from a reference plane at the boundary between the main surfaceand the chamfered surface. This is caused by that the flagging in theend shape is increased by mechanical polishing for a very short timewhich was conducted for decreasing the surface roughness of the glasssubstrate caused by the plasma etching, and caused by difficulties inprecise measurement of the peripheral shape of the substrate.

INDUSTRIAL APPLICABILITY

Deformation of a substrate at the periphery can be decreased by applyingthe present invention to a reticle having a pattern-formed thin film, toa substrate for a mask blank before the forming of a pattern, and to amethod manufacturing a reticle or a substrate.

1. A reticle substrate comprising a pair of main surfaces opposing each other, two pairs of side faces that are right to the main surfaces and that are opposed in pair to each other, and chamfered surfaces between the main surfaces and the side faces, wherein: at least one of the main surfaces has a flatness not greater than 0.5 μm on a flatness measuring area which is laid inside of an intermediate area of 3 mm located within a boundary between the at least one main surface and the chamfered surface; the boundary between the at least one main surface and the chamfered surface has a maximum height between −1 and 0 μm (excluding 0 μm) from a reference surface determined in relation to the flatness measuring area disposed inside of the intermediate area; and the intermediate area between the flatness measuring area and the chamfered surface is declined or lowered from the flatness measuring area towards the chamfered surface.
 2. A mask blank comprising a thin film for a transfer pattern, on the at least one of the main surfaces of the reticle substrate according to claim
 1. 3. The mask blank according to claim 2, wherein the thin film has a film stress of 0.5 Gpa or less.
 4. The mask blank according to claim 2 or 3, wherein the flatness is not greater than 0.5 μm on a flatness measuring area of the main surface on which the thin film is formed, with the intermediate area of 3 mm which is laid inwardly from the boundary between the main surface and the chamfered surface and which is exempted from the flatness measuring area, while the boundary between the main surface and the chamfered surface has the maximum height between −1 and 0 μm from the reference surface determined in relation to the flatness measuring area laid inside of the intermediate area; and wherein: the intermediate area between the flatness measuring area and the chamfered surface is lowered from the flatness measuring area towards the chamfered surface.
 5. A method for manufacturing the reticle substrate according to claim 1, characterized by the steps of: grinding and precisely polishing a main surface of a reticle substrate; thereafter measuring a surface configuration of the main surface on an area that includes a substrate peripheral portion to be supported by a substrate-holding member of an exposure apparatus; and modifying the surface configuration of the main surface on the basis of a result of the measurement so that the surface configuration of the main surface becomes a desired shape, by finding that an area of the main surface is convex relative to an optionally determined reference plane, by providing, on the area, a pressure higher than the other areas onto polishing pads of a polishing apparatus, with a polishing liquid being supplied towards the polishing pads, and by moving the reticle substrate relative to the polishing pads.
 6. The method for manufacturing the reticle substrate according to claim 5, wherein the above-mentioned precisely polishing step comprises: a roughly polishing process of removing surface defects of the substrate while maintaining the flatness obtained in the grinding process by using a relatively large abrasive grain; and a mirror-polishing process of polishing the surface of the substrate for mirror finish by using a relatively small abrasive grain.
 7. A method for manufacturing the mask blank by forming a thin film as a transfer pattern on a main surface of the reticle substrate manufactured by the method according to claim 5 or
 6. 8. The method for manufacturing the mask blank according to claim 7, comprising: a heating process of suppressing that change of the maximum height from the reference plane which appears during or after the forming of the thin film, at the boundary between the main surface and the chamfered surface, the heating process being carried out before and after the forming of the thin film.
 9. A reticle substrate comprising a pair of main surfaces opposing each other, two pairs of side faces right to the main surfaces wherein the two side faces of each pair opposing each other, and chamfered surfaces between the main surfaces and the side faces, wherein: at least one of the main surfaces has a flatness not greater than 0.5 μm on a flatness measuring area which is laid inside of an intermediate area of 3 mm located within a boundary between the main surface and the chamfered surface; while a boundary between the flatness-measuring area and a flatness non-measuring area has a maximum height between −1 and 0 μm from a reference surface determined in relation to the flatness measuring area laid inside of the intermediate area; and wherein: the intermediate area between the flatness measuring area and the chamfered surface is lowered from the flatness measuring area towards the chamfered surface.
 10. A mask blank comprising a thin film functioning as a transfer pattern formed on at least one of the main surfaces of the reticle substrate according to claim
 9. 11. The mask blank according to claim 10, wherein the thin film has a film stress of 0.5 Gpa or less.
 12. The mask blank according to claim 10 or 11, wherein the flatness is not greater than 0.5 μm on a flatness measuring area of the main surface on which the thin film is formed, with the intermediate area of 3 mm which is laid inwardly from the boundary between the main surface and the chamfered surface and which is exempted from the flatness measuring area, while the boundary between the flatness measuring area and the flatness non-measuring area has the maximum height between −1 and 0 μm from the reference surface determined in relation to the flatness measuring area laid inside of the intermediate area; and wherein: the intermediate area between the flatness measuring area and the chamfered surface is lowered from the flatness measuring area towards the chamfered surface.
 13. A method for manufacturing the reticle substrate according to claim 9, characterized by the steps of: grinding and precisely polishing a main surface of a reticle substrate; thereafter measuring a surface configuration of the main surface on an area that includes a substrate peripheral portion to be supported by a substrate-holding member of an exposure apparatus; and modifying the surface configuration of the main surface on the basis of a result of the measurement so that the surface configuration of the main surface becomes a desired shape, by finding that an area of the main surface is convex relative to an optionally determined reference plane, by providing, on the area, a pressure higher than the other areas onto polishing pads of a polishing apparatus, with a polishing liquid being supplied towards the polishing pads, and by moving the reticle substrate relative to the polishing pads.
 14. The method for manufacturing the reticle substrate according to claim 13, wherein the above-mentioned precisely polishing step comprises: a roughly polishing process of removing surface defects of the substrate while maintaining the flatness obtained in the grinding process by using a relatively large abrasive grain; and a mirror-polishing process of polishing the surface of the substrate for mirror finish by using a relatively small abrasive grain.
 15. A method for manufacturing the mask blank by forming a thin film as a transfer pattern on a main surface of the reticle substrate manufactured by the method according to claim 13 or
 14. 16. The method for manufacturing the mask blank according to claim 15, comprising: a heating process of suppressing that change of the maximum height from the reference plane which appears during or after the forming of the thin film, at the boundary between the main surface and the chamfered surface, the heating process being carried out before and after the forming of the thin film.
 17. The reticle substrate according to claim 1, wherein: the flatness measuring area has a flatness not greater than 0.25 μm while the boundary between the main surface and the chamfered area has a maximum height between −0.5 μm and 0 μm relative to the reference surface.
 18. The reticle substrate according to claim 9, wherein: the flatness measuring area has a flatness not greater than 0.25 μm while a boundary between the flatness measuring area and the flatness non-measuring area has a maximum height between −0.5 μm and 0 μm relative to the reference surface.
 19. The method for manufacturing the reticle substrate according to claim 5, wherein the modifying step is carried out so that a desired relationship could be accomplished between a flatness of the flatness measuring area and a declined degree of the intermediate between the flatness measuring area and the chamfered area.
 20. The method for manufacturing the reticle substrate according to claim 13, wherein the modifying step is carried out so that a desired relationship could be accomplished between a flatness of the flatness measuring area and a declined degree of the intermediate between the flatness measuring area and the chamfered area. 